1. Field of the Invention
The present invention relates to a method of producing dislocation-free silicon single crystals with a [110] crystallographic axis orientation by the Czochralski method (hereinafter referred to as CZ method).
2. Description of the Related Art
During growing a silicon single crystal by the CZ method, the heat shock incurred when a silicon seed crystal is brought into contact with silicon melt causes dislocations to occur within the silicon seed crystal. The dislocations thus generated in those silicon single crystals with a [100] crystallographic axis orientation which are in general and frequent use can be made to disappear in the necking process.
However, from the viewpoint of crystal structure, a silicon single crystal with a [110] crystallographic axis orientation has a [111] plane which is a slip plane parallel to the axial pull-up direction, and the dislocations generated in the silicon seed crystal upon contacting with the silicon melt hardly run out of the silicon seed crystal in the necking process. Therefore, the dislocations are carried over to the grown crystal via a neck section, causing the problem that no dislocation-free silicon single crystals can be grown.
In addition, for producing silicon single crystal ingots larger in diameter and heavier in weight, it is necessary to increase the neck diameter and, from this point of view as well, it has become increasingly difficult to remove dislocations.
Various technologies have so far been developed and proposed for rendering silicon single crystals grown by the CZ method free of dislocations. Among them, Japanese Patent No. 3,726,847 discloses a method of producing silicon single crystals by the CZ method according to which, to cope with the increases in the diameter and weight of silicon single crystals, use is made of a silicon seed crystal having a boron concentration of not less than 2.7×1017 atoms/cm3 and not more than 1.4×1019 atoms/cm3, the melt with the high concentration of boron added likewise is used as silicon melt and a neck section diameter of 5-8 mm is employed.
It is alleged that, according to this method, the dislocations generated at the time of contact of the silicon seed crystal with the silicon melt are immobilized and prevented from propagating by the high-concentration dopant in the silicon seed crystal and, thus, the neck section can be rendered dislocation-free even when the neck section diameter is increased and, with the result that the neck section strength is improved to thereby make it possible to pull up a large-diameter, heavy-weight silicon single crystal.
Japanese Patent Application Publication No. 09-249492 describes a method of pulling up silicon single crystals using silicon seed crystals having a boron concentration within the range of from 5×1019 to 6×1020 atoms/cm3. The high concentration of boron increases the stress level required for the dislocations generated upon contact of the silicon seed crystal with the melt to propagate towards the upper part of the silicon seed crystal as compared with the ordinary boron concentration, whereby the dislocations can be inhibited from propagating. Accordingly, when the portion where the dislocations have been introduced can be removed by dissolving the same in the melt and, allegedly, it becomes possible to pull up a single crystal on the basis of the resulting dislocation-free silicon seed crystal.
Further, Japanese Patent Application Publication No. 04-139092 discloses a method of producing silicon single crystals using silicon seed crystals containing at least 1×1019 atoms/cm3 of boron to thereby increase the yield strength of the silicon seed crystal to a level not less than the thermal stress acting at the moment of contact of the silicon seed crystal with the silicon melt and thus reduce the extent of dislocations in the silicon seed crystal.
The methods described in those documents, namely Japanese Patent No. 3,726,847, Japanese Patent Application Publication No. 09-249492 and Japanese Patent Application Publication No. 04-139092, all consist in adding a high concentration of boron to a silicon seed crystal to inhibit the propagation of dislocations or the formation of dislocations and thereby render the silicon seed crystal dislocation-free or inhibit the propagation of dislocations, and thus allow a dislocation-free single crystal to grow from the resulting seed crystal surface.
However, when attempts were made to grow dislocation-free silicon single crystals with a diameter of 300 mm and a [110] crystallographic axis orientation using silicon seed crystals doped with a high concentration of boron, it was not always possible to pull up dislocation-free silicon single crystals in good yield and in a stable manner, without dropping or the fear of dropping.
On the other hand, Japanese Patent No. 3,555,081 describes a method of producing silicon single crystals doped with arsenic or antimony and having a crystal conical part (corresponding to a transitional region from the neck section to the cylindrical crystal) with a subtended/included angle of 40°-60°. However, the dislocation-free single crystals obtainable by this method are the ones with a (100) surface orientation and the included angle cannot be said to be large; thus, the method has drawbacks that a long period of time is required for the formation of the conical part and the productivity is thus low.